Method of impregnating pressed metal articles



METHOD OF IMZPREGNATIN G PRESSED METAL ARTICLES James H. Lum, Dayton, Ohio, assignor to Monsanto Chemical Company, a corporation of Delaware No Drawing. Application February 20, 1941,

Serial No. 379,833

Claims. (CI. 75-22) This invention relates to metallic articles such as may be produced by the processes of powder metallurgy and particularly provides such articles in the form of substantially non-porous products.

It is an object of this invention to provide a pressed and sintered metal article of increased strength and durability and one which may be employed in the production of fittings for conveying fluids under pressure.

Another object is the provision of metal articles of decreased porosity which shall have and retain a surface adaptable to receive paints, lacquers, enamels, etc.

In the processes of powder metallurgy it is the usual practice to prepare metals in the form of finely divided powders either by comminuting metals or by reduction of suitable reducible oxides of metals, by decomposition of carbonyls of certain of the metals or by electrolytic processes. Various metals which may be prepared in the powdered form and which may be useful for the present purpose are iron, copper, aluminum, zinc, brass, nickel, cobalt, etc. Such metal powders may be molded directly in suitable dies by the application of pressure or mixtures of the above powdered metals may be utilized for the production of alloyed metallic articles for various uses. For the production of pressed articles, it is desirable that a fairly soft metal be employed, such for example, as a carbon-free iron which may also be free of other alloying elements. In cases where it is desired to utilize metal powders made from relatively brittle metals I may add to the molding powder suitable bonding agents such as the softer metals or alloys or organic bonding agents for temporarily bonding the same.

Articles pressed or briquetted from metallic powders are quite porous and generally lack sufflcient strength for useful service and therefore it is customary to sinter such molded articles by subjecting them to temperatures suitable for causing a regrowth or recrystallization of the metal crystals thereby to strengthen the structure. The tempertatures utilized are dependent upon the particular metal employed for producing the'pressed article and are also dependent upon the time of sintering. In the case where carbon-free powdered iron is employed the sintering temperature may range from in the neigh-' borhood of 1,800 to 1,900 F. upwardly to 2,000 to 2,100 F. or more, which temperature is preferably applied to the article while the same is maintained in a non-oxidizing atmosphere.

The article thus obtained is quite strong and generally not brittle. However, even though fairly high pressures are employed in the preliminary formation of the article, it possesses an undesirable degree of porosity which seriously limits its application for many uses, particularly for the production of fittings such as pipe fittings, used for conveying fluids under pressure. Such porosity may be in the neighborhood of to by volume where iron powder is used and will vary with the fineness and nature of the metal powder employed, the pressure and the temperature of sintering.

I have now found that such pressed articles as above described may be treated with vinyl compounds, capable of polymerizing to the solid state while said vinyl compounds are still in the fluid nonpolymerized or partly polymerized state. Such application may be made by merely dipping the pressed article into a fluid monomeric vinyl compound orthe treatment may be carried out by placing the article in a closed container which may then be evacuated of air, thereby removing substantially all of the air from the pores of the pressed article. While thus under a vacuum I may run in the fluid monomer or partially prepolymerized body and thereupon readmit atmospheric air to the chamber causing the monomer to penetrate the pores of the pressed article. I may also apply additional pressure in excess of atmospheric to efiect a further penetration.

In particular where low boiling monomers or mixtures are employed for impregnation, it is desirable to employ higher pressures so that the pores of the metal briquette are substantially completely filled with the liquid-monomer. An example is the case of vinyl chloride which as a monomer boils at a. temperature of about -14 C., while under atmospheric pressure, but which may be liquefied by application of a suitable pressure thereon.

The impregnated article is then removed from the chamber. and may readily be freed of excess vinyl compound upon the surface merely by evaporation or by wiping with a cloth after which the artice may be heated in a vapor-proof system if necessary to prevent loss of the vinyl compound through evaporation, to a temperature effective to polymerize the compound.

Among those vinyl compounds or substituted vinyl compounds which I may use. for the present purpose are styrene, vinyl chloride, vinyl acetate, vinyl carbazole, vinylidene chloride, esters of acrylic acid, such as methyl acrylate, esters of methacrylic acid, such as methyl methacrylate, acrylonitrile and mixtures of the above.

In place of mixtures of vinyl compounds I may employ mixtures of vinyl compounds with other unsaturated organic compounds capable of copolymerizing with said vinyl compounds,'for example styrene with maleic anhydride, vinyl acetate with diallyl adipate, styrene with triallyl phosphate, styrene with butadiene, etc.

Vinyl compounds generally, after polymerization, have been found to satisfactorily fill up the 'm pores residing in the pressed metallic briquette so as to completely prevent the leakage of liquids through the impregnated metal article even though such liquids are under considerable pressure. Moreover pipe fittings produced by means of my invention are resistant to the action of water and also mineral oils such as lubricating oil.

When strong solvents are employed in contact with the impregnated metal article I may employ mixture of compounds which during polymerization will produce a cross-linked polymer which is highly insoluble in solvents generally. Examples of compounds effectively resistant to solvent action are styrene copolymerized with divinyl benzene, styrene with dimethallyl maleate, vinyl acetate with diallyl adipate, etc.

Where high temperatures, i, e. of the order of 200 to 300 F. are employed I may employ styrene alone or mixed with other compounds or I may employ vinyl carbazole in like manner.

The above vinyl compounds or mixtures thereof with other compounds when employed in the manufacture of pipe fittings such as elbows, unions, Ts, or even pipe itself present a surface which requires little or no preparation prior to painting lacquering or enameling and by virtue of the substantially non-porous structure thus obtained remains adherent over long periods of service. During polymerization of the vinyl compounds enumerated, there is no liberation of gases or vapors beyond a slight volatilization of monomer during the natural polymerization reaction and hence the surface of the article is not impaired nor are voids produced by the escape of reaction products or of solvents.

In many cases I may find it desirable to emp oy in conjunction with my polymerizable compounds, various plasticizers, stabilizers, etc. to obtain various effects. It is also desirable to employ polymerization accelerators, i. e., catalysts such as organic peroxides. in particular benzoyl peroxide, which enable me to shorten the time of polymerization.

What I claim is:

The method of fabricating metallic articles f"om powders comprising briquetting a mass of metallic powder by means of pressure to produce a porous briquette, sintering said briquette at a temperature sufiicient to eil'ect at least a. partial recrystallization of said particles, cooling said briquette and thereupon treating said briquette with a monomeric vinyl compound to effect substantial penetration of said vinyl compound into said pores, thereupon heating said monomeric vinyl compound at a temperature and for a period of time to effect a polymerization thereof in said acres.

2. The method of fabricating non-porous shaped iron articles from iron powder comprising briquetting a mass of metallic powder by means of pressure exerted upon a mass of said iron powder enclosed in a die conforming to the shape of said article, to produc a porous briquetted article sintering said brlquetted article at a temperature sufllcient to effect at least a partial recrystallization of iron particles, cooling said article and thereupon treating said briquette with fluid monomeric styrene to eflect a substantial penetration of said fluid styrene and heating said article to eifect a polymerization of said styrene.

3. The method of fabricating metallic articles from powders comprising briquetting a mass of metal powder by means of pressure to produce a porous briquette, sintering said briquette at a temperature suflicient to eflfect at least a partial recrystallization of said particles, thereupon treating said briquette with a fluid monomeric vinyl compound mixed with a cross linking agent to effect at least a partial penetration of said vinyl compound into said pores, thereupon heating said impregnated briquette under conditions to prevent substantial volatilization of vinyl compound, at a temperature and for a period of time to effect a polymerization of said vinyl compound.

4. The method of fabricating non-porous metal articles from metal powders, comprising briquetting a mass of metallic powder by means of pressure to produce a porous briquette, sintering said briquette at a temperature suflicient to effect at least a partial recrystallization of said particles, cooling said briquette and thereupon treating said briquette with fluid monomeric styrene to efiect a substantial penetration of said styrene into said pores, thereupon heating said im pregnated briquette at a temperature and for a period of time to effect polymerization of said styrene in said pores.

5. A process which comprises treating a porous metallic briquette with a monomeric vinyl compound to effect penetration of the vinyl compound into the pores of said briquette and polymerizing said compound in situ.

6. A process which comprises treating a porous metallic briquette with a mixture of a monomeric vinyl compound and a cross linking agent to effect penetration of the pores of the briquette and polymerizing the mixture in situ.

7. A process which comprises treating a porous metallic briquette with a mixture of a monomeric vinyl compound and a cross linking agent from the group consisting of divinyl benzene, maleic anhydride, diallyl adipate, triallyl phosphate, butadiene and dimethallyl maleate, to effect penetration of said mixture into the pores of said briquette and polymerizing the mixture in situ.

8. A process which comprises introducing a porous metallic briquette into a closed evacuated chamber to remove the air from the pores of the matrix, running in a fluid monomeric vinyl compound, readmitting atmospheric air to the chamber to cause the monomer to penetrate the pores of the briquette, removing the briquette from the chamber and heating it to polymerize the vinyl compound in situ.

9. A process according to claim 5 in which superatmospheric pressure is employed in the impregnating step.

10. A process according to claim 5 in which vinyl chloride is the monomer employed.

JAMES H. LUM. 

